This application discloses a pressure relief valve of the type usable on vessels for containing highly corrosive chemicals. Such valves may be used on railway tank cars or other transportation or storage vessels.
For various reasons, railway industry standards set by the AAR currently require that, aside from O-rings and other seals, the materials for construction of pressure relief valves be metallic. For example, parts connected directly to the vessel wall may undergo substantial bending or tensile stresses in use. Typically, the parts of such pressure relief valves which may be xe2x80x9cwettedxe2x80x9d by, i.e., exposed to contact with, the lading in use, are formed of metals exhibiting high corrosion resistance, such as stainless steel, zirconium, and alloys such as those sold under the trademarks MONEL and HASTELLOY, all of which are quite costly. Furthermore, parts made of such metals heretofore have had to be fabricated by casting, resulting in significant porosity. While some materials, such as zirconium have recently become available in less porous bar stock, it is still quite expensive.
Less costly materials which afford adequate corrosion resistance, such as certain plastics, are known, but have heretofore not been permitted in pressure relief valves for railway tank cars because they have lacked sufficient tensile or bending strength and/or fire-resistance to be used in the xe2x80x9cwettedxe2x80x9d parts of current pressure relief valve designs.
In prior pressure relief valve designs, the main valve body has been of unitary one-piece metal construction. Thus, while only portions of such valve bodies may be exposed to corrosive chemicals in use, repair of corrosion damage has entailed replacing the entire body, or returning the valve to the manufacturer for remachining and/or recoating of the valve body, since such procedures cannot be performed in the field.
This application discloses a pressure relief valve which avoids the disadvantages of prior valve designs, while affording additional structural and operating advantages.
An important aspect is the provision of a pressure relief valve for use in highly corrosive environments which is of simpler and more economical construction that current designs.
Another aspect is the provision of a pressure relief valve of the type set forth, which has parts exposed to highly corrosive ladings in use which can, nevertheless, be safely formed of non-metallic materials.
In connection with the foregoing aspect, another aspect is the provision of a pressure relief valve of the type set forth, wherein the parts of the valve exposed to corrosive ladings are subjected to minimized tensile or bending stresses.
A still further aspect is the provision of a pressure relief valve of the type set forth, wherein the parts exposed in use to corrosive ladings are relatively easily field-replaceable.